The present invention relates to digital communication systems and more particularly to systems for transmitting using Orthogonal Frequency Division Multiplexing (OFDM).
Modern digital communication systems typically incorporate a preliminary step of translating data to be transmitted into a series of so-called symbols. Each symbol can take on one of M possible complex values and therefore carries log2M bits of information. The symbols define modulation on a sinusoidal carrier with the real part of each symbol defining a in-phase component of the modulated carrier and the imaginary part of each symbol defining a quadrature (90 degrees shifted relative to in-phase) component of the modulated carrier.
The generation of symbols is a digital process whereas the ultimately transmitted signal is typically analog. In a wireless system, the ultimately transmitted signal is at the so-called xe2x80x9cRFxe2x80x9d or radio frequency. Since the radio frequency is often quite high rendering certain signal conditioning steps more difficult, it is common to first modulate a signal at a lower frequency called the xe2x80x9cIFxe2x80x9d or intermediate frequency and then convert this IF signal to the RF frequency.
One way to modulate the IF signal is to isolate the real and imaginary components of the symbols, multiply the real portion by a digital cosine waveform at the IF frequency and the imaginary portion by a sine waveform at the IF frequency. The resulting two scalar digital signals are then converted to analog and summed to produce the IF signal. This approach has been found to be impractical because even very tiny deviations from 90 degrees in the phase difference between the cosine and sine waves cause unacceptable crosstalk between the real and imaginary symbol components as captured at the receiver end.
A more robust modulation technique relies on a system design where the IF frequency is selected to be, e.g., greater than two times the baseband sampling frequency. An upsampler increases the sampling rate of the baseband symbol stream by a factor of L by interspersing Lxe2x88x921 zero values between each baseband symbol. This has the effect of replicating the baseband symbol stream spectrum in the frequency domain. An interpolation filter selects the baseband component in the frequency domain. This component is shifted to an IF frequency. In the time domain, the real part of the shifted signal is extracted. Conversion of this real part to analog produces the analog IF signal. Although interpolation filter design may be difficult, this is a commonly used solution.
There are, however, difficulties with this technique when it is applied to so-called xe2x80x9cOFDMxe2x80x9d communication systems. The abbreviation xe2x80x9cOFDMxe2x80x9d refers to Orthogonal Frequency Division Multiplexing, a highly useful technique for developing time domain symbols for transmission. In OFDM, the available bandwidth is effectively divided into a plurality of subchannels that are orthogonal in the frequency domain. During a given symbol period, the transmitter transmits a symbol in each subchannel. To create the baseband time domain signal corresponding to all of the subchannels, an IFFT is applied to a series of frequency domain symbols to be simultaneously transmitted, a xe2x80x9cburst.xe2x80x9d The resulting series of time domain symbols is augmented with a cyclic prefix prior to transmission. The cyclic prefix addition process can be characterized by the expression:
[z(1) . . . z(N)]T[z(Nxe2x88x92v+1) . . . z(N)z(1) . . . z(N)]T
OFDM provides particularly good performance in systems subject to multipath effects where different copies of the same symbol may be received at different times due to their travel by differing length paths. Because OFDM greatly increases the length of the symbol period, these multipath effects are experienced as self-cancellation or reinforcement of the same symbol rather than intersymbol interference.
The cyclic prefix has length v where v is greater than or equal to a duration of the impulse response of the overall channel and assures orthogonality of the frequency domain subchannels. The overall channel, however includes any filtering used in the modulation and upconversion process and thus includes the interpolation filter. To effectively select only the baseband frequency domain component of the upsampled time domain signal typically requires an FIR with many taps or an IIR, either of which will greatly increase the time duration of the channel impulse response. The increased duration of the channel impulse response requires a longer cyclic prefix which reduces data carrying efficiency.
The filtering operations required in OFDM systems are even more challenging because individual OFDM bursts follow one another in succession in the time domain. The abrupt transitions between bursts give rise to spurious components that lie outside the defined frequency domain subchannels. These components should be greatly attenuated 1) to prevent the resampling processes from creating further spurious artifacts within the frequency domain signal and 2) to prevent out-of-band spurious emissions that exceed government requirements.
What is needed are systems and methods for accurately converting a baseband OFDM signal to an IF signal without substantially lengthening of the channel impulse response duration experienced by the OFDM signal while sufficiently attenuating spurious out-of-band artifacts.
Systems and methods for converting a baseband OFDM signal to an IF signal while minimizing lengthening of the impulse response duration experienced by the OFDM signal are provided by virtue of the present invention. A conversion technique according to the present invention provides sufficient filtering to limit the effects of spurious frequency domain components caused by transitions between successive OFDM bursts. In one embodiment, the filtering is provided by a combination of a finite impulse response (FIR) filter having non-linear phase characteristics and a cyclic convolution filter. Conversion from the frequency domain into the time domain, upsampling, and cyclic filtering may be combined into one operation.
In accordance with a first aspect of the present invention, apparatus for transmitting OFDM signals includes: a transform processor that transforms a frequency domain burst of symbols into a time domain burst of symbols, and a signal processing system that receives the time domain burst of symbols and performs operations on the time domain burst of symbols to develop a filtered time domain burst of symbols. The signal processing system includes an FIR filter having non-linear phase characteristics.
In accordance with a second aspect of the present invention, apparatus for transmitting OFDM signals includes: a transform processor that transforms a frequency domain burst of symbols into a time domain burst of symbols, and a signal processing system that receives the time domain burst of symbols and performs operations on the time domain burst of symbols. The signal processing system includes a cyclic convolutional filter that provides a filtered time domain symbol burst as output.
In accordance with a third aspect of the present invention, apparatus for transmitting OFDM signals includes: a transform processor that transforms a series of frequency domain symbol bursts into time domain symbol bursts; and a filtering system that digitally filters the time domain symbol bursts to ameliorate spectral artifacts caused by interburst boundaries while minimizing delay spread induced by the filtering system.
A further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings.